The three members of the p160 steroid receptor coactivator (SRC) family enhance gene transcription mediated by nuclear receptors and certain other transcription factors. During the last funding cycle, we extensively studied the in vivo functions of these coactivators by using genetic mouse models. Our published works have firmly established the importance and essential pleiotropic roles of the SRC family in many physiological and pathogenetic processes. We found that SRC family members regulate development, somatic growth, vascular protection, reproduction and endocrine function. Furthermore, SRC family members promote breast and prostate cancer initiation and progression. Of particular note, genetic ablation of SRC3 in mice inhibits prostate cancer progression, arresting tumors in a well-differentiated state. While SRC3 is expressed in the basal cells (BCs) but not expressed in the normal exocrine luminal epithelial cells (ELECs), it is overexpressed in the advanced prostate tumors. In line with the stem cell (SC) concept for prostate epithelial homeostasis and our preliminary studies, we hypothesize that SRC3 is expressed in the basal SCs, intermediate cells (IMCs) and cancer stem cells (CSCs); SRC3 silencing is required for ELEC differentiation; and persistant SRC3 expression is required for deviation and maintenance of CSCs as well as androgen-independent tumor cells. To test our hypothesis, we will pursue three specific aims. The first aim is to define the SRC3 expression pattern in each cell type of prostate epithelium and its determinant role in maintenance of prostate epithelial homeostasis in adult animal. We will address if SRC3 is expressed in SCs, IMCs, CSCs and neuroendocrine cells (NECs) of the prostate epithelium, and whether ectopic SRC3 expression in ELECs will affect proliferation, differentiation and transformation. The second aim is to define the role of SRC3 in derivation and maintenance of CSCs during prostate cancer initiation and progression. We will develop an inducible gene targeting system to specifically inactivate floxed SRC3 alleles in the putative stem/precursor cell lineage to investigate if SRC3 deficiency in this cell lineage will inhibit CSC formation and prostate cancer progression. The third aim is to investigate whether SRC3 expression in prostate cancer cells results from SRC3 induction in SRC3-negative tumor cells and to determine if this SRC3 induction promotes prostate cancer progression. We will conditionally knock out SRC3 in the ELEC lineage and test if this will prevent SRC3 expression in prostate tumor cells and prostate cancer progression. These studies will elucidate the cell lineage-specific origin of SRC3-positive prostate cancer cells as well as the cell type-specific role of the SRC3 proto- oncoprotein in prostate cancer progression. The knowledge to be obtained from these proposed studies may suggest SRC3 as a cell lineage-specific and tumor stage-specific target for prostate cancer treatment.